Conversion of biomass, including plants, animals, and their organic waste products and residue, currently satisfies approximately fourteen percent of the world's energy needs. Unused, discarded biomass residues from forestry, agriculture, and municipal sources are potential energy resources which at present are not well managed and thus pose significant environmental problems. More effective uses of these resources for bio-energy and related bio-products could help reduce fossil fuel emissions and mitigate greenhouse gas emissions implicated in climate change.
Current methods for converting biomass into energy include combustion, pyrolysis, gasification, biological conversion to ethanol, and biological conversion to gas including methane and (more recently) hydrogen. Combustion is a convenient method of extracting energy from biomass sources, but it can cause significant health problems associated with the production and release of soot and other chemicals into the air. Gasification may be more efficient and healthier than the direct combustion of biomass, but this process requires very high temperatures and specialized equipment. More recently, biomass containing cellulosic fiber has been converted into ethanol, but this process requires significant energy and expensive enzymes or gasification to produce ethanol. Non-cellulosic, starch- or sugar-derived ethanol is easier and less expensive to create, but it simultaneously creates pressures on food sources while not improving carbon emissions.
Anaerobic digestion (fermentation) is a well-known, low energy requiring system. Anaerobic digestion has traditionally been used in the wastewater treatment industry to reduce both the volume and the organic content of waste sludge. As anaerobic digestion technologies have evolved, the process has been applied to the treatment of a wide range of high-strength liquid waste, as well as various solid organic wastes, to reduce the chemical oxygen demand of waste effluent or to divert solid waste from disposal, and to produce bioenergy containing a mixture of methane and carbon dioxide. Conventional anaerobic digestion of organic matter results in a biogas consisting primarily of methane (60-75%) and carbon dioxide (25-40%).
Hydrogen is a promising alternative clean energy source. For example, hydrogen is not considered a greenhouse gas and it is more economical than methane at less than stoichiometric yields. While significant amounts of hydrogen can be produced from biomass feedstocks using anaerobic digestion under conditions that favor hydrogen production, a major problem in mixed anaerobic fermentation is that produced hydrogen is rapidly consumed by acetogenic and methanogenic bacteria, which convert hydrogen to acetate and methane. Thus, due to rapid “hydrogen consumption” and low hydrogen yield during anaerobic digestion, hydrogen gas has been considered only as a process control index or indicator of organic shock loading in the process of methane production.
Previous attempts to solve the problems associated with capturing hydrogen in mixed anaerobic fermentation have largely relied on a two reactor vessel system to separate hydrogen production from methane production. Such two-reactor vessel systems remove the hydrogen as it is being formed to prevent its consumption and conversion to methane. The costs of operating such a bioreactor system are dependent on the size and complexity of the system. The capital cost of the bioreactor itself is significant. Additionally, previous efforts have required the purification and/or pretreatment of bacterial colonies to enrich hydrogen-producing species and/or sterilization of the biomass feedstock to avoid hydrogen consumption, each of which adds significant capital costs and operating costs. For example, pretreatment of bacterial colonies to enrich for hydrogen-producing bacteria requires an external reaction step in a separate reactor vessel from the anaerobic digestion reactor, contributing to the cost of such approaches.
Thus, there is still an on-going need for improved methods for producing hydrogen and methane as alternative clean energy sources.